Conventionally, identification seals carrying hologram and other marks that cannot be readily forged have been affixed to articles such as credit cards, monetary papers, tickets and commercial goods to discourage any attempts to illicitly duplicate them.
These identification seals are normally intended to be visually identified, and are therefore placed in highly visible places. These identification seals are thus so visible that it is relatively easy for a potential forger to analyze the structure of the identification seals. Furthermore, forgery of such identification seals is relatively easy because it suffices if the forged identification seal can successfully deceive the eye of the user.
It was also proposed to use an optical reader to identify an identification seal. Examples of such article identification systems are found in the description of a number of commonly assigned United States patents, and for more details of this technical field reference should be made, for instance, to U.S. Pat. Nos. 5,300,764 issued Apr. 5, 1994, 5,291,006 issued Mar. 1, 1994, and 5,200,794 issued Apr. 6, 1993. Because the identification process is carried out by a machine, the accuracy of identification is improved as compared to the naked eye, and forgery can be prevented even more effectively. However, it still is possible for a potential forger to locate the position of the identification region, and to analyze it with a certain amount of effort.